A wireless power transmitting system for wirelessly transmitting power between coils based on a coupling coefficient or a mutual inductance of the coils are known. A greater product of a coupling coefficient k between the coils and Q factors of the coils leads to a higher maximum value of transmission efficiency between the coils.
To raise the coupling coefficient k, a resonator structure in which a coil is wound around a magnetic core is proposed. With such a structure, it is possible to provide the wireless power transmission in which reduction of the coupling coefficient is small with respect to a positional displacement between the transmitting and receiving resonators in a direction parallel to that of the winding of the coil, and which is robust against efficiency variations.
In a wireless power transmission in a resonance method, transmission efficiency becomes maximum when the resonance frequencies of both transmitting and receiving resonators are equal. However, the resonance frequency of a resonator for wirelessly transmitting power varies under a variety of conditions such as shape, ambient environment, and long-term deterioration, which may cause a problem of the resonance frequency deviation from a desired value.
For example, in the above resonator structure, when the distance between a power transmitting coil and a power receiving coil is short, an inductance value is increased under the influence of a magnetic core on the counterpart side. The inductance value also varies depending on the positional relation between the transmitting and receiving resonators. These things cause a problem of the resonance frequency deviation from the desired value, leading to deterioration of the transmission efficiency.